Substrate and fetch affect the emergence of freshwater mussels from lake sediments

Unionid mussels are among the largest and longest-lived invertebrates in shallow benthic communities, where they often make up most of the biomass. However, they spend their juvenile stages and extended periods of their adult lives buried in the sediments and emerge from the sediments only during part of the year. I tested whether the magnitude and timing of emergence of Elliptio complanata, an abundant native mussel, are related to water temperature, upwelling activity, site exposure, and sediment characteristics. Seasonal changes in the density and average body size of mussels on the sediment surface were measured at 26 shallow (2-m) littoral sites in a multibasin lake to determine which factors are most useful in predicting their emergence. In general, densities started increasing in early spring, peaked in spring or sometime in summer, and declined through autumn, but the magnitude (ratio of minimum-to-maximum surface density) and timing (date of maximum surface density, start of autumn decline) of emergence differed between sites. Maximum mussel surface density was on average (median) 2X higher than minimum surface density and was higher at exposed sites with fine sediment particles than at more protected sites. The timing of mussel emergence and burial were most closely related to site exposure. Mussels reached their maximum surface density earlier, but they also started burying earlier at the most exposed sites (effective fetch > 900 m). At more protected sites, maximum densities were reached later in summer, after the main period of reproduction, and densities did not start declining until late summer. The only exceptions were 2 protected sites with deep, fine, highly organic and, presumably, anoxic sediments, where mussels emerged early in the spring and did not bury until late in the season. The timing of mussel emergence varied greatly in different parts of the littoral zone. These results suggest that the functional role of unionid mussels in shallow benthic communities might change seasonally and might change differently at sites exposed to different levels of physical disturbance.